The present invention pertains to the preparation of thermosettable ethenyl (vinyl) terminated polymers or prepolymers.
Japanese Patent No. Sho 42[1967]-14468 discloses that the reaction product of pyridine having methyl substituents with pyromellitic dianhydride produced a polymer having a melting point above 300.degree. C. Copending application Ser. No. 754,699, filed July 15, 1985, discloses that the reaction product of pyrazine having alkyl substituents with mono- or dianhydrides produced a polymer having a melting point at, below and above 300.degree. C. Both polymers are cured by a condensation reaction which generates water. This water vaporizes during cure and produces undesirable voids and/or surface imperfections in the composite or other cured article.
It has now been discovered that these undesirable characteristics of the cured articles can be reduced or eliminated by employing the thermosettable polymers of the present invention. These ethenyl terminated prepolymers of the present invention can be cured by an addition reaction via the unsaturated terminal groups, thereby eliminating the release of water vapor and resulting in composites or other cured articles which do not possess surface imperfections. Cured ethenyl terminated prepolymers prepared by reacting 2,3,5,6-tetramethylpyrazine, pyromellitic dianhydride and a material containing a polymerizable ethylenically unsaturated group provides 16-18% higher char yield in nitrogen at 950.degree. C. than prepolymers prepared by reacting 2,3,5,6-tetramethylpyrazine with a mono- or dianhydride. Char yield is defined herein as the percent by weight of the polymer remaining after exposure to 950.degree. C. during a thermogravimetric analysis of the polymer in a nitrogen atmosphere.
The ethenyl terminated prepolymers of the present invention are also suitable as novel comonomers for reaction with N,N-bis-imide resins. The copolymerization of the ethenyl terminated prepolymers with N,N-bis-imide resins occurs at a lower cure temperature than that of a N,N'-bis-imide resin alone, which results in energy conservation. Since the copolymerization is an addition reaction, volatile generation is minimized.